Seals and fyke nets: An investigation of the problem and its possible solution

During the last few decades, the economically important eel (Anguilla anguilla) fishery with fyke nets along the Swedish west coast has been subjected to damage caused by harbour seals (Phoca vitulina). To protect fyke nets from seals, the netting in the fish bags was replaced with stronger material. The level of damage decreased when fishing with modified fyke nets, compared with when traditional fyke nets were used. The stronger fyke nets were, however, still exposed to a notable level of attack from seals, even if the resulting damage was minor compared with that suffered by the standard fyke nets. There was no difference in the catch per unit effort when comparing the most effective modified fyke nets with the standard fyke nets. By testing exclusively modified fyke nets in certain areas, and only standard fyke nets in other fishing areas some distance away, the levels of damage were reduced in the areas with only modified fyke nets. By replacing traditional fyke nets with modified fyke nets, fishermen can make it less profitable for seals to use fyke nets as food stores, with less gear damage and zero catch losses as a result. Using modified fyke nets in the commercial eel fishery is both practical and rewarding for the fishermen.     

Assessment and control of non‐indigenous brown bullhead Ameiurus nebulosus populations using fyke nets in shallow ponds

In order to evaluate fyke nets as a tool for population assessment and mass removal of brown bullhead Ameiurus nebulosus populations, small, shallow ponds were sampled during late summer in 1999 and 2000. Catch efficiency, species and size selectivity of fyke nets were investigated with the mark‐recapture method. Two different strategies were applied: (1) on six occasions, the mark‐recapture method was applied to entire populations, and (2) in six other cases, sampling was done in two replicated enclosed areas (625 m²). When applied to entire populations, fyke nets were found to be size selective, and capture efficiency significantly increased with standard length. Conversely, no size selectivity of fyke nets was found in enclosed areas. Sampling in replicated enclosures yielded reproducible estimates of population density and biomass. The results suggest that double fyke nets, when combined with the mark‐recapture technique, are a very useful tool for the efficient and reliable assessment of brown bullhead populations. The results of the study also suggest that double fyke nets may potentially be a cost‐effective tool for the mass removal of non‐indigenous brown bullhead populations.     

Biological monitoring of fish and crustaceans in the Wadden Sea — potential and problems

Monitoring of fish and crustaceans in the Wadden Sea (WS) must cope with rapid changes in distribution patterns, access to certain areas and gear efficiency. Application and limitations of a variety of fishing devices (fyke nets, gill nets, enclosures, stow nets, purse seines, beam trawls, push nets, beach seines, bottom trawls, pelagic trawls) are discussed with regard to different objectives of monitoring. Furthermore, the validity of data from three current monitoring programmes is also discussed. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1–4 November 1988)     

Assessment of burbot Lota lota (L. 1758) population sustainability in central European reservoirs

A novel sampling scheme, using a combination of electrofishing, visual exploration by scuba divers, two types of fyke nets and longlines, was tested in four reservoirs (including their inlets and outlets) to monitor a population of burbot Lota lota. This was supplemented by fry trawling and vertical hydro‐acoustics, to detect L. lota larvae in two deep reservoirs that have had a long‐term stocking programme. The majority of the L. lota detected were juveniles, captured by electrofishing in the littoral zones of the reservoirs and in running waters. Older individuals were rarely captured with longlines or fyke nets in deeper zones or structured habitats within the reservoirs. A combination of multiple sampling methods provided an assessment of the whole population. Population establishment could not be demonstrated as the age structure of the sampled fish corresponded with that of the stocked fish. Low post‐stocking survival, migratory behaviour, interactions with other species and warmer water temperatures are considered the potential drivers for unsuccessful establishment of L. lota populations in these reservoirs.     

A Novel Tool to Mitigate By-Catch Mortality of Baltic Seals in Coastal Fyke Net Fishery

Developing methods to reduce the incidental catch of non-target species is important, as by-catch mortality poses threats especially to large aquatic predators. We examined the effectiveness of a novel device, a “seal sock”, in mitigating the by-catch mortality of seals in coastal fyke net fisheries in the Baltic Sea. The seal sock developed and tested in this study was a cylindrical net attached to the fyke net, allowing the seals access to the surface to breathe while trapped inside fishing gear. The number of dead and live seals caught in fyke nets without a seal sock (years 2008–2010) and with a sock (years 2011–2013) was recorded. The seals caught in fyke nets were mainly juveniles. Of ringed seals (Phoca hispida botnica) both sexes were equally represented, while of grey seals (Halichoerus grypus) the ratio was biased (71%) towards males. All the by-caught seals were dead in the fyke nets without a seal sock, whereas 70% of ringed seals and 11% of grey seals survived when the seal sock was used. The seal sock proved to be effective in reducing the by-catch mortality of ringed seals, but did not perform as well with grey seals.     

Benthic community response to a passive fishing gear in a coastal lagoon (South Brazil)

The influence of a passive shrimp fishing gear on benthic communities was studied at Laguna Estuarine System (South Brazil), a shallow choked coastal lagoon. The gear is composed by a group of fyke nets (25 mm mesh size) set in contact to the bottom, fixed with stakes forming a cage-like structure (around 30 m²). Samplings were conducted in the two main fishery areas of the estuarine system, Mirim (sand bottoms) and Imaruí (muddy bottoms) lagoon, in May 2005. In each area, 10 fyke net enclosures and 10 nearby sites without nets (control) were sampled. Microphytobenthos biomass (chlorophyll a and phaeopigments), number of taxa/species, density, Hill’s number N ₁ and N ₂, and estimated number of species (ES₁₀₀) were used as community attributes. For the nematodes, values of the maturity index and abundance of Wieser’s feeding type were used as well. The effects of the small-scale passive shrimp fishing gear on the coastal lagoon bottoms were dependent on the benthic component analyzed and the type of sediment. Whereas macrofauna was not affected by the net enclosures, meiofauna and nematodes, particularly from the mud sites were. At the sand site, the fyke net enclosures caused a decrease in the microphytobenthos biomass and changed the relative abundances of non-selective deposit feeding and epigrowth-feeding nematodes. The results indicated that small-scale static nets, such as the studied fyke enclosures, produced low intensity levels of disturbance. However, the enclosed area by nets at Laguna had already reached around 25,000 m². Given the large proportion of the coastal population involved and the area closed by nets, management policies should consider site-specific differences within the same estuarine system.     

Control of invasive American bullfrog Lithobates catesbeianus in small shallow water bodies

Setting up cost-efficient control programs for alien invasive species requires the development of adequate removal methods in combination with insights in population size and dynamics. American bullfrog Lithobates catesbeianus is an alien invasive species, which is suspected to cause substantial ecological damage around the globe. However, control of bullfrog populations is difficult, as no conclusive management measures have yet been determined. We investigated how double fyke nets could contribute to bullfrog management by assessing the tadpole population size in 10 permanent small shallow water bodies. Two population size estimate methods were applied, being the catch–depletion and mark–recapture method. Catchability of bullfrog tadpoles proved to be very consistent over ponds and methods, with one catch per unit of effort (one double fyke net for 24 h) retaining on average 6 % of the tadpole population. Population density varied considerably among ponds, ranging from 950 to 120,804 larger tadpole individuals/ha. Using these insights in developing a cost-efficient eradication program for the species, we projected the number of catch efforts needed to reduce tadpole numbers to a threshold that more than likely affects final bullfrog population size. Predictions indicated that for the specified thresholds the use of eight double fyke nets at a time is most cost-efficient in high abundance populations, while using five double fyke nets seems most suitable in low abundance populations. What the exact threshold number of remaining tadpole individuals should be is uncertain, but forecasts demonstrate that only half of the budget would be needed when aiming at a drop to fewer than 100 remaining tadpoles than when a decrease to fewer than 10 remaining tadpoles is pursued. Given the fairly limited cost of bullfrog management with double fyke nets, however, it may be worthwhile to fully reduce the tadpole population.     

Using new electrofishing technology to amp‐up fish sampling in estuarine habitats

A prototype, boat‐mounted electrofisher capable of operation in estuarine waters (where electrical conductivities often exceed 20 000 µS cm^?1) was assessed. Electrofishing was compared to fyke and mesh netting in four riverine estuaries and to seining in a lagoonal estuary (consisting of a series of brackish coastal lakes separated from the sea by a barrier system of sand dunes). Fish assemblage composition, length distributions and the probability of detecting ecological fish guilds (relating to estuary use, position in the water column and body size) were compared among gears. The assemblage composition of electrofishing samples differed from those of fyke nets in all riverine estuaries and from mesh netting in two. The assemblage composition of electrofishing samples differed from those of seining in structured seagrass habitats of the lagoonal estuary. When all species were pooled, the electrofisher sampled a broader range of lengths than either fyke or mesh netting in riverine estuaries or seining in lagoonal estuaries. The bias of electrofishing and netting towards particular species and size classes affected the probability of detecting some ecological guilds, highlighting the potential implications of gear choice on understanding estuarine ecological function. The detection of guilds varied with gear type and environmental conditions, including stratification, water depth and surface electrical conductivity. Assessments with the aim to characterize the structure of fish assemblages will benefit from the use of multiple gears. Electrofishing shows immense promise for discretely sampling highly structured habitats to test hypotheses about their use.     

The population density, growth rate and production of bream, Abramis brama, in Tjeukemeer, the Netherlands

A method of estimating the population density of bream in Tjeukemeer (21.3 km²) using 16 690 introduced fish (fin-clipped and opercular tagged) is described. Gill nets of the winter fishery proved to be a more effective method of sampling the population for marked fish than fyke nets. The population density of bream (⋝25 cm) was estimated to be 180 000. There was no significant difference between the estimates derived from fin-clipped and opercular tagged fish. The growth rate of bream in Tjeukemeer (L_∞=41 cm) is poor compared with that of bream in other waters, due to its high density and the scarcity of zoobenthos available to it.
The production of bream (I–XV) is estimated to be 34 kg ha⁻¹ of which 25% is contributed by two (1959, 1963) strong year classes. Of a total biomass of 37.5 kg ha⁻¹ available to the fishery in 1969 only 2.7 kg ha^−l was removed as yield. The average P/B ratio for the population was low (0.39). It is concluded that a major flow of energy to bream is through zooplankton.     

Fish Assemblages in Three Tidal Saltmarsh and Mangrove Flats in Temperate NSW, Australia: A Comparison Based on Species Diversity and Abundance

Fish assemblages in three geographically distinct saltmarsh meadows and mangrove forests in the Sydney region were examined using fyke nets over three common sampling periods. The saltmarshes at Towra Point and Allens Creek, though of contrasting geomorphic setting, showed similar fish assemblages during spring tides, with relatively high diversity and abundance. The saltmarsh at Bicentennial Park, reclaimed from dredge spoil in the early 1960’s, showed significantly lower diversity and abundance. Fish assemblages in the three mangrove settings were each distinct, though with no significant differences in diversity and abundance between sites. The result raises questions about the efficacy of created saltmarsh as a compensatory habitat for fish.     

Length‐weight relationships of nine fish species from the Qingjiang River,Yangtze River basin,China

This study presents the first report of length‐weight relationships (LWRs) for five fish species (Parabotia fasciata, Megalobrama mantschuricus, Plagiognathops microlepis, Rhodeus sinensis, Rhodeus fangi) and maximum total lengths for four species from the Qingjiang River, Yangtze River basin, China. Specimens were sampled in six sections along the Qingjiang River over a period of 1 year between April 2016 and March 2017 with gill nets (150 × 10 m; mesh size: 1–14 cm), benthic fyke nets (mouth opening: 12 × 12 cm; 10 m; mesh size: 1 cm) and stationary lift nets (10 × 10 m; mesh size: 0.8 cm).     

Fish assemblages across a vegetation gradient in a restoring tidal freshwater wetland: diets and potential for resource competition

Marsh habitats have been the focus of recent worldwide restoration efforts due to their degradation and destruction as a result of human development. We assessed fish resource use at a naturally restoring marsh (Liberty Island, California, USA) by comparing diet composition, stomach fullness, normalized stomach weight, and diet overlap across a vegetation gradient. Fish were collected using gill nets and fyke nets at six sites during spring 2010, summer 2011 and winter 2012. We analyzed 392 stomachs from the eleven most abundant species collected. Prey composition and biomass varied seasonally for all fish species, but there were no notable differences across sites or seasons for stomach fullness or normalized stomach weight for most fish species. Results from non-metric multidimensional scaling (NMDS) and two-way analysis of similarities (ANOSIM) indicated minimal diet overlap between species (R?=?0.633, p?=?0.001) and seasons (R?=?0.413, p?=?0.001). Seasonal habitat and resource use across the vegetation gradient was species-specific. Small but significant spatial diet differences were detected for inland silverside, striped bass and bigscale logperch. Delta smelt exhibited seasonal diet differences by shifting from chironomids and zooplankton during spring, to amphipods and zooplankton during winter. More generally, fish maintained stomach fullness across all sites and seasons while maintaining minimal dietary overlap. Our study emphasizes the importance of tidal marshes as feeding habitat for several fish species, including the endangered delta smelt.     

Influence of water temperature and net tending frequency on the condition of fish bycatch in a small-scale inland commercial fyke net fishery

To date, most studies of commercial fisheries bycatch have focused on mortality at time of capture as an endpoint. However, sub-lethal indicators of organismal condition have the potential to reveal mechanisms associated with mortality (both at time of capture and post-release) and opportunities for improving fish welfare. In this study, we simulated commercial fishing efforts in inland lakes with fyke nets during a typical fishing season (early April to late June) in southeastern Ontario, Canada, where bycatch of non-target fish species had previously been documented. Using non-target gamefish (i.e., largemouth bass [Micropterus salmoides, Lacépède], northern pike [Esox Lucius, L.]), as well as a target species (i.e., bluegill [Lepomis macrochirus, Rafinesque]), we examined the sub-lethal consequences of capture (e.g., blood physiology, reflex impairment, and injury) and compared the effects of being retained in the net for two different durations (i.e., two or six days) over a range of water temperatures (i.e., 3–28 °C). Sub-lethal physiological disturbances (i.e., blood glucose and lactate) in largemouth bass and bluegill tended to be greater at higher water temperatures. However, fish retained for six days generally did not exhibit greater stress than those retained for two days, with the exception of plasma glucose in largemouth bass. Reflex impairment was similar among temperature and retention periods. Fish retained in nets experienced a range of injuries (including fin frays, scale loss, and mouth damage) that had the potential to facilitate the development of opportunistic pathogenic infections. Greater incidences of injury on fish bycatch tended to be associated with higher temperatures and longer retention. To reduce physiological disturbances and injury that could lead to delayed mortality, we suggest that regulations for inland commercial fishers require them to check their nets more frequently as water temperatures increase. We suggest that future studies of bycatch incorporate sub-lethal endpoints given that they serve as an objective measure of fish welfare and can provide quantitative mechanistic information to support management actions.     

Utilization of a citizen monitoring protocol to assess the structure and function of natural and stabilized fringing salt marshes in North Carolina

Narrow fringing salt marshes dominated by Spartina alterniflora occur naturally along estuarine shorelines and provide many of the same ecological functions as more extensive marshes. These fringing salt marshes are sometimes incorporated into shoreline stabilization efforts. We obtained data on elevation, salinity, sediment characteristics, vegetation and fish utilization at three study sites containing both natural fringing marshes and nearby restored marshes located landward of a stone sill constructed for shoreline stabilization. During the study, sediment accretion rates in the restored marshes were approximately 1.5- to 2-fold greater than those recorded in the natural marshes. Natural fringing marsh sediments were predominantly sandy with a mean organic matter content ranging between 1.5 and 6.0%. Average S. alterniflora stem density in natural marshes ranged between 130 and 222 stems m⁻², while mean maximum stem height exceeded 64 cm. After 3 years, one of the three restored marshes (NCMM) achieved S. alterniflora stem densities equivalent to that of the natural fringing marshes, while percentage cover and maximum stem heights were significantly greater in the natural than in the restored marshes at all sites. There was no significant difference in the mean number of fish, crabs or shrimp captured with fyke nets between the natural and restored marshes, and only the abundance of Palaemonetes vulgaris (grass shrimp) was significantly greater in the natural marshes than in the restored ones. Mean numbers of fish caught per 5 m of marsh front were similar to those reported in the literature from marshes adjacent to tidal creeks and channels, and ranged between 509 and 634 fish net⁻¹. Most of the field data and some of the sample analyses were obtained by volunteers as they contributed 223 h of the total 300 h spent collecting data from three sites in one season. The use of fyke nets required twice as many man-hours as any other single task. Vegetation and sediment parameters were sensitive indicators of marsh restoration success, and volunteers were capable of contributing a significant portion of the labor needed to collect these parameters. The U.S. Government's right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.     

Diel variation in fish assemblages in tidal creeks in southern Brazil.

Tidal creeks are strongly influenced by tides and are therefore exposed to large differences in salinity and depth daily. Here we compare fish assemblages in tidal creeks between day and night in two tidal creeks in southern Brazil. Monthly day and night, simultaneous collections were carried out in both creeks using fyke nets. Clupeiformes tended to be caught more during the day. Cathorops spixii, Genidens genidens and Rypticus randalli tended to be caught at night. Sciaenidae also tended to be caught more during the night. In general, pelagic species were diurnal, while deep water species were nocturnal. These trends are probably due to a variety of causes, such as phylogeny, predation and net avoidance.     

Use of seasonal freshwater wetlands by fishes in a temperate river floodplain

This study examined the use of freshwater wetland restoration and enhancement projects ( i.e . non-estuarine wetlands subject to seasonal drying) by fish populations. To quantify fish use of freshwater emergent wetlands and assess the effect of wetland enhancement ( i.e . addition of water control structures), two enhanced and two unenhanced emergent wetlands were compared, as well as two oxbow habitats within the Chehalis River floodplain. Eighteen fish species were captured using fyke nets and emigrant traps from January to the beginning of June, with the most abundant being three-spined stickleback Gasterosteus aculeatus and Olympic mudminnow Novumbra hubbsi . Coho salmon Oncorhynchus kisutch was the dominant salmonid at all sites. Enhanced wetlands, with their extended hydroperiods, had significantly higher abundances of yearling coho salmon than unenhanced wetlands. Both enhanced and unenhanced emergent wetlands yielded higher abundances of non-game native fishes than oxbow habitats. Oxbow habitats, however, were dominated by coho salmon. Fish survival in the wetland habitats was dependent on emigration to the river before dissolved oxygen concentrations decreased and wetlands became isolated and stranding occurred. This study suggests that wetland enhancement projects with an outlet to the river channel appear to provide fishes with important temporary habitats if they have the opportunity to leave the wetland as dissolved oxygen levels deteriorate.     

Changes in marsh nekton communities along the salinity gradient of the Schelde river, Belgium and The Netherlands

Nekton was sampled in five marshes along the salinity gradient of the Schelde River. The utilisation of three different habitats (large and small creek, marsh pond) by fish and macrocrustacean species was compared among the five sampling sites. In the larger channels fyke nets were deployed to capture fish and macrocrustaceans leaving the marsh at ebb while block nets were set in smaller intertidal creeks. Fish traps passively sampled fish and shrimp in the marsh ponds. The tidal freshwater marsh had a species poor fauna and only a low number of fish was caught. Besides some freshwater species (Alburnoides bipunctatus, Carassius carassius) the European eel, Anguilla anguilla was still present. The four other marshes had a similar community structure although Platichthys flesus was absent from the euhaline area. Among fish species, dominance of Dicentrarchus labrax, Platichthys flesus and Pomatoschistus microps was observed. Carcinus maenas and Palaemonetes varians were the most abundant macrocrustacean species in every marsh. Between the large and small intertidal creeks there was no difference in nekton species composition. The main species used both habitats. Marsh ponds were utilized intensively only by two species, Pomatoschistus microps and Palaemonetes varians in every marsh.     

Length‐weight relationships of three freshwater fish species from the Nandu River and Changhua River in Hainan Island,China

Length‐weight relationships (LRWs) were determined for three fish species from the Hainan Island, China. Fish specimens were sampled across the Nandu River and Changhua River from May 2017 to June 2018 with benthic fyke‐nets (stretched mesh size: 0.4 cm), multi‐mesh gillnets (stretched mesh size ranging from 1.0 to 12.5 cm) and electrofishing. Relationships between length and weight were fitted very well for all species (all R²>0.95). The b values of the LRW equations ranging from 2.938 for Liniparhomaloptera qiongzhongensis to 3.259 for Toxabramis houdemeri.     

Comparison of Day and Night Fyke Netting, Electrofishing and Snorkelling for Monitoring a Population of the Threatened Golden Galaxias (Galaxias auratus)

The littoral zone of small off-stream water storage containing a translocated population of Galaxias auratus was sampled fortnightly at day and night with fyke nets, electrofishing and snorkelling over 3 months. Variation in population data provided by each method, including relative abundance indices, size structure, and habitat preferences, were examined. Aspects of behaviour and activity patterns were also investigated. Night sampling using all methods consistently yielded larger catches than day sampling. The size structure of catches varied, with electrofishing at night and fyke netting during the day having higher proportions of juveniles, whilst snorkelling at night and electrofishing during the day had higher proportions of adults. Fyke netting at night yielded by far the largest catches (~3-fold more than other methods) and also captured balanced proportions of juveniles and adults. Galaxias auratus had a strong diel activity pattern and were most active at night. The majority of the population migrated into the littoral zone during the night and back into deeper water during the day. A small number of juveniles remained in the littoral zone and some adults sheltered in the dense cover of species-rich littoral vegetation during the day. Shores with shallow depth profiles appeared to be preferred due to higher catches in these areas using all methods. Based on the results of this study, fyke netting at night in littoral habitats is recommended for monitoring populations of G. auratus. Fyke netting is likely to be an effective method for monitoring other lacustrine galaxiid species; however, further work is required to investigate the effects of habitat variables and fish community structure on activity patterns of galaxiids, and hence their catchability with various methods, in more extensive lentic environments.     

Rapporteur's report: sources and interaction mechanisms

This, the first session of the Workshop, focussed on sources of exposure to and interaction mechanisms of intense static magnetic fields. The four presentations dealt with (1) the magnetic fields produced by diagnostic magnetic resonance imaging systems; (2) the ways in which static magnetic fields can interact with matter, in particular human tissue; (3) the effects of magnetic fields on enzyme reaction rates; and (4) working practices for high-field magnet development.